A stock market was an organisation of people who bought and sold stocks.
In October 1929, the Wall Street stock market crashed, triggering the Great Depression across Earth. (TV: Daleks in Manhattan)
Ferran read the Eighth Doctor's memory, and saw — among other things — stock market crashes as an example of evil on Earth. (PROSE: Father Time)
The Sixth Doctor foiled a plot by the Master and the Usurians to take over the stock market and destabilise the Earth's economy. (PROSE: Business Unusual)